Injection compression molding method and injection compression machine of lens

ABSTRACT

An injection compression molding method of a lens is provided, where a toggle link mechanism ( 65 ) is actuated to close a molding die ( 50 ) and a movable die plate ( 64 ) is moved to a position establishing a cavity thickness of greater than a thickness of an article to be molded while the die is closed. After injecting a molten resin into the cavity, the molten resin is sealed in the cavity and the toggle link mechanism ( 65 ) is actuated to advance the movable die plate ( 64 ) toward a fixed die plate ( 61 ), the relative position of a rear die plate ( 62 ) and the movable die plate ( 64 ) is made constant at a position where extension of a tie bar ( 63 ) becomes a predetermined value, and the molten resin is cooled for a predetermined time after completion of pressurizing the resin.

This is a Division of application Ser. No. 10/281,174 filed Oct. 28,2002. The disclosure of the prior application is hereby incorporated byreference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to injection compression molding methodand machine of a lens for injecting and compressing thermoplastic resinto mold a lens. More specifically, it relates to an injectioncompression molding method and an injection compression molding machineof a lens capable of reducing sink mark and strain, restraining strainof molding article, and easily changing the thickness of the moldingarticle, which is suitable for molding a spectacles-lens.

2. Description of Related Art

Conventionally, an injection compression molding method is known as amethod for molding spectacles lenses using thermoplastic resin.

In the injection compression molding method, in order to compensateshrinkage of molten resin to obtain uniform and high shape accuracy,molding die is clamped while retaining compression margin inside thespectacles-lens molding cavity, the molten resin is injected and filledin the spectacles-lens molding cavity, the molding die is compressed bythe compression margin and the molten resin is cooled to obtain thespectacles-lens (see Japanese Patent Laid-Open Publication No. Hei9-277327 and Japanese Patent Laid-Open Publication No. Hei 9-216263).

Since a meniscus lens used as a spectacles-lens is a thick moldingarticle having thickness difference between the central portion andperipheral portion thereof, the cooling time has to be taken long inorder to restrain the sink mark, shrinkage deformation, internal strainetc, which lengthen molding cycle.

Japanese Patent Laid-Open Publication No. Hei 9-234774 discloses acooling time of 230 seconds for molding a minus lens (a lens havingthicker peripheral portion than the central portion) of 76 mm diameterand lens power of −4.00 D (diopter).

In the above cooling step, the molten resin compressed in the moldingdie clamped by a predetermined clamping force loses resin pressurethereof inside the molding die in accordance with molding shrinkage dueto cooling. The resin pressure gradually decreases in accordance withthe progress of cooling and, after completion of cooling, the moldingarticle is released from the molding die during ejecting step.

In order to obtain a lens with high accuracy, it is important torestrain the sink mark, shrinkage deformation, internal strain anddeformation during die-releasing step as well as raising thetransferability for accurately transferring the concave and convex shapeof the spectacles-lens molding cavity to the lens. Accordingly, acooling step for uniformly and sufficiently cooling the entire moltenresin filled in the spectacles-lens-molding cavity is required.

In the conventional injection compression molding method, the moltenresin is cooled for a predetermined time while keeping the pressureapplied to the resin at a predetermined level during the cooling step,and the molding die is rapidly parted to reduce the pressure applied tothe resin instantly. Accordingly, deformation is likely to be caused onthe molding article during the die-releasing step if the cooling time isnot sufficient.

Further, there can be partial parting (peeling) between the dies and themolding article according the manner for opening the dies during thedie-releasing step, which causes parting failure of so-called“separation”.

Further, the lens thickness has to be changed in accordance with lenscharacteristics, e.g. lens power (such as spherical power and astigmaticpower) in molding a spectacles-lens including finished lens andsemi-finished lens.

In the conventional method, since the lens is obtained by compressingthe molding die for a predetermined compression margin, i.e., since thecavity is reduced to a position for giving a desired thickness of finalproduct, in order to change the thickness of a lens, a molding diehaving a cavity of final thickness of molding article has to be preparedfor each lens and the molding article has to be changed in accordancewith the lens to be molded.

Accordingly, a plurality of types of molding dies have to be preparedand much production cost is necessary, and additional work is requiredfor exchanging the molding die, thereby deteriorating productionefficiency.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an injectioncompression molding method of a lens and an injection compressionmolding machine capable of reducing sink mark and strain and shorteningmolding cycle.

An injection compression molding method of a lens according to an aspectof the present invention is for molding a lens of a thermoplastic resin,the method comprising the steps of: providing a molding machine having atie bar for mutually connecting a pair of space-retaining plates, amovable die plate movable along the tie bar, a molding die providedbetween a first space-retaining plate of the pair of the space-retainingplates and the movable die plate and accommodating a lens-molding cavityincluding a pair of cavity forming members thereinside for shapingconcave and convex surface of the lens, and an advancement-retractionmechanism provided between a second space-retaining plate of thespace-retaining plates and the movable die plate for advancing andretracting the movable die plate relative to the first space-retainingplate, the lens-molding cavity being reduced when the movable die plateadvances toward the first space-retaining plate while the molding die isclosed; actuating the advancement-retraction mechanism to close themolding die and to move the movable die plate to a position for thethickness of the lens-molding cavity to be thicker than a thickness of amolding article while the molding die is closed; injecting athermoplastic molten resin into the lens-molding cavity defined whilesetting the volume of the cavity and sealing in the molten resin insidethe molding die; actuating the advancement-retraction mechanism toadvance the movable die plate toward the first space-retaining plate,keeping a constant relative position between the second space-retainingplate and the movable die plate where the extension of the tie barbecomes a predetermined value in accordance with the characteristics ofthe lens and pressurizing the molten resin; and cooling the molten resinfor a predetermined time after completion of pressurizing the resin.

According to the above injection compression molding method of a lens,the molding die is closed by actuating the advancement-retractionmechanism during the cavity volume setting step, and the movable dieplate is moved to a position where the thickness of the lens-moldingcavity becomes greater than the thickness of the article to be moldedwhile the die is closed.

Subsequently, during resin injection seal-in step, the molten resin isinjected into the lens-molding cavity. Since the thickness of thelens-molding cavity is enlarged greater than the thickness of thearticle to be molded, inappropriate resin resistance against the moldingdie is not generated and injection and filling of the resin can besmoothly conducted during injection of the molten resin. The moltenresin injected into the cavity is sealed in the molding die. In otherwords, the molten resin is prevented from returning (backflowing) to theoutside of the molding die.

Next, during resin pressurizing step, the movable die plate is advancedtoward the first space-retaining plate by actuating theadvancement-retraction mechanism. At this time, since the molten resinis filled and sealed in the molding die including the lens-moldingcavity, the relative position between the second space-retaining plateand the movable die plate is kept constant at a position where the tiebar is extended and the extension of the tie bar becomes a predeterminedvalue. Then, the reaction force of the extension of the tie bar works onthe molten resin inside the lens-molding cavity through the movable dieplate. In other words, the molten resin inside the lens-molding cavityis pressurized by the reaction force of the tie bar extension.Incidentally, the resin pressurizing step may be started after,simultaneously with, or immediately before completion of injecting andfilling the resin.

Lastly, during the cooling step, the molten resin is cooled for apredetermined time after completion of the resin pressurizing step.Then, the molten resin inside the lens-molding cavity is cooled whilebeing compressed, which is gradually solidified and shrunk in accordancewith progress of the cooling. The movable die plate gradually advancesin accordance with the shrinkage of the resin. When the movable dieplate advances, the space between the pair of the space-retaining platesare reduced, i.e. the elasticity of the tie bar is recovered, therebygradually lowering the pressure applied to the molten resin inside thelens-molding cavity.

Thus, in accordance with the solidification and shrinkage of the moltenresin inside the lens-molding cavity being gradually cooled, thepressure applied to the molten resin inside the lens-molding cavity canbe gradually lowered through the advancement of the movable die plateand the recovery of the tie bar elasticity. Accordingly, the entirety ofthe obtained molding article can be uniformly cooled, so that the sinkmark and strain can be reduced and cooling time, the entire moldingcycle as well, can be reduced. For reference, the die-releasing step canbe started after cooling time of eighty seconds for molding a lens of 76mm diameter and lens power of −4.00 D (diopter) according to the methodof the present invention.

In the above, the molding die may preferably have a fixed die fixed tothe first space-retaining plate and having one of the pair of the cavityforming members constituting the lens-molding cavity thereinside and amovable die fixed to the movable die plate, and the movable die maypreferably have a die body having the other of the pair of the cavityforming member constituting the lens-molding cavity thereinside, a dieattachment fixed to the movable die plate and movably holding the diebody relative to the fixed die, and a resilient member interposedbetween the die attachment member and the die body, the die attachmentmember and the die body being capable of being opened and closed by apredetermined gap by the resilient member.

Accordingly, during the cavity volume setting step, the fixed die andthe movable die are closed by actuating the advancement-retractionmechanism and, while the dies are closed, the movable die plate isadvanced to move the die attachment member and the die body toward eachother while compressing the resilient member, so that the thickness ofthe lens-molding cavity can be set at a predetermined thickness greaterthan the thickness of the article to be molded. In other words, only byadvancing the movable die plate by actuating the advancement-retractionmechanism, the molding die can be closed and the thickness of thelens-molding cavity can be set at the predetermined thickness while themolding die is closed.

In the above, a toggle link mechanism may preferably be used as theadvancement-retraction mechanism, the toggle link mechanism beingstretched to close the molding die and the distance between the pair ofspace-retaining plates being adjusted so that the extension of the tiebar becomes a predetermined value when the toggle link mechanism ismoved to an extension limit thereof to establish a minimum volume of thelens-molding cavity, thereby adjusting a clamp force, and the togglelink mechanism may preferably be actuated to the extension limit thereofwhile pressurizing the resin.

Accordingly, since a clamping force adjusting step where the toggle linkmechanism is stretched to close the molding die and the distance betweenthe pair of space-retaining plates is adjusted so that the extension ofthe tie bar becomes a predetermined value when the toggle link mechanismis moved to an extension limit thereof to establish a minimum volume ofthe lens-molding cavity is provided, the operation during the resinpressurizing step can be facilitated.

Specifically, by adjusting the distance between the pair ofspace-retaining plates so that the extension of the tie bar becomes apredetermined value (ΔL, for instance) when the toggle link mechanism isadvanced to the extension limit thereof to set the minimum volume of thelens-molding cavity, it is only necessary to drive the toggle linkmechanism to the extension limit during the resin pressurizing step. Theextension of the tie bar during the resin pressurizing step is equal tothe sum of the extension (ΔL) during the clamping force adjusting stepand the extension (α) of the tie bar due to incomplete closing of themolding die on account of the resin volume inside the lens-moldingcavity, so that it is only necessary to drive the toggle link mechanismto the extension limit during the resin pressurizing step after settingthe extension (ΔL) considering the extension (α) during the clampingforce adjusting step.

The molding die is designed so that the thickness of the lens-moldingcavity of the molding die becomes smaller than the thickness of anarticle to be molded when the toggle link mechanism is moved to theextension limit thereof.

In the above, the second space-retaining plate may preferably be movedtoward and away from the first space-retaining plate for adjusting theclamp force.

Ordinarily, a die thickness adjuster for adjusting the space between thespace-retaining plates by moving one of the space-retaining platestoward and away from the other is provided to an injection compressionmachine, and the second space-retaining plate can be moved toward andaway from the first space-retaining plate using the die thicknessadjuster.

In the above, a shutting member may preferably be projected into a spruein communication with the lens-molding cavity through a runner to shut anozzle channel for injecting the molten resin after injecting the moltenresin into the lens-molding cavity.

According to the above arrangement, since an operation for projectingthe shutting member to the nozzle channel is only required, when themolten resin is completely injected into the lens-molding cavity, theinjected molten resin can be immediately sealed therein. Therefore, evenwhen the resin pressurizing step is started immediately beforecompletion of injection, the resin backflow can be securely prevented.

Another object of the present invention is to provide an injectioncompression molding method of a lens capable of restraining thedeformation of the molding article during die-releasing step.

An injection compression molding method of a lens according to anotheraspect of the present invention is for molding a lens of a thermoplasticresin, the method comprising: providing step for providing a moldingmachine having a tie bar for mutually connecting a pair ofspace-retaining plates, a movable die plate movable along the tie bar, amolding die provided between a first space-retaining plate of the pairof the space-retaining plates and the movable die plate and having afixed die and a movable die moving toward and away from the fixed die,the fixed die and the movable die respectively having a cavity formingmember for shaping concave and convex surface of the lens, the cavityforming member of the fixed die and the cavity forming member of themovable die forming a lens-molding cavity, and an advancement-retractionmechanism provided between a second space-retaining plate of thespace-retaining plates and the movable die plate for advancing andretracting the movable die plate relative to the first space-retainingplate, the lens-molding cavity being reduced when the movable die plateadvances toward the first space-retaining plate while the molding die isclosed; cavity-volume setting step for closing the molding die andmoving the cavity forming member of the movable die to a position wherethe thickness of the lens-molding cavity becomes a predeterminedthickness greater than the thickness of an article to be molded; resininjection seal-in step for injecting a thermoplastic molten resin intothe lens-molding cavity established in setting the volume of the cavityand sealing the molten resin injected to the lens-molding cavity; resinpressurizing step for advancing the cavity forming member of the movabledie toward the cavity forming member of the fixed die to compress themolten resin injected into the lens-molding cavity; cooling step forcooling the molten resin for a predetermined time after completion ofpressurizing the resin; and die-releasing step comprising a primarydie-release operation for lowering the pressure applied to the moltenresin inside the lens-molding cavity while keeping substantiallyconstant relative position between the cavity forming member of themovable die and the cavity forming member of the fixed die for apredetermined time after completion of cooling the molten resin and anactual die-release operation for opening the movable die relative to thefixed die.

In the above, in the die-releasing step, the pressure applied to themolten resin inside the lens-molding cavity is lowered after completionof the cooling step while the relative position between the cavityforming member of the movable die and the cavity forming member of thefixed die is kept substantially constant during the primary die-releaseoperation, and the movable die is opened relative to the fixed die. Inother words, when the pressure applied to the resin inside thelens-molding cavity is lowered during the primary die-releasing step,the relative position between the cavity forming member of the movabledie and the cavity forming member of the fixed die is kept substantiallyconstant, so that the deformation of the molding article can berestrained.

Since the relative position between the cavity forming member of themovable die and the cavity forming member of the fixed die is keptsubstantially constant in spite of the fact that the pressure applied tothe resin is lowered, the deformation of the molding article during thedie-releasing step can be reduced even when the cooling step isshortened, thereby obtaining a highly accurate lens molding article.Accordingly, highly accurate lens can be obtained and the molding cyclecan be shortened as well.

In the above, during the cooling step, the pressure applied to themolten resin inside the lens-molding cavity may preferably be graduallylowered at a speed slower than the pressure reduction during thedie-releasing step while keeping substantially constant relativeposition between the cavity forming member of the movable die and thecavity forming member of the fixed die for a predetermined time.

According to the above arrangement, since the pressure applied to themolten resin inside the lens-molding cavity is gradually lowered at aslow rate during the cooling step while the cavity forming member of themovable die and the cavity forming member of the fixed die is keptsubstantially constant, the obtained molding article can be uniformlycooled. As a result, sink mark or strain can be reduced, and the coolingtime, entire molding cycle as well, can be reduced.

In the above, the pressure applied to the molten resin inside thelens-molding cavity may preferably be lowered while keeping the changein the space between the cavity forming member of the movable die andthe cavity forming member of the fixed die within a difference betweenthe thickness of the lens-molding cavity and the thickness of thearticle to be molded and ejected at least during the primary die-releaseoperation of the die-releasing step in the die-releasing step and thecooling step.

The difference between the thickness of the lens-molding cavity duringcompression and the thickness of the ejected molding article ordinarilydiffers according to the characteristics of the lens material and thelens power.

Specifically, the change in the space between the cavity forming memberof the movable die and the cavity forming member of the fixed die maypreferably be kept not more than 0.3 mm, preferably not more than 0.2mm, more preferably not more than 0.1 mm. When the change in the spacebetween the cavity forming member of the movable die and the cavityforming member of the fixed die is greater than the above value,deformation of the molding article is likely.

The arrangement of the second aspect of the present invention may becombined with the arrangement of the first aspect of the presentinvention.

Further object of the present invention is to provide an injectioncompression molding method of a lens capable of easily changing thethickness of the lens with low cost and requiring no special additionalwork.

An injection compression molding method of a lens according to thirdaspect of the present invention is for molding a lens of a thermoplasticresin, the method comprising the steps of: providing a molding machinehaving a tie bar for mutually connecting a pair of space-retainingplates, a movable die plate movable along the tie bar, a molding dieprovided between a first space-retaining plate of the pair of thespace-retaining plates and the movable die plate and having a fixed dieand a movable die moving toward and away from the fixed die, the fixeddie and the movable die respectively having a cavity forming member forshaping concave and convex surface of the lens, the cavity formingmember of the fixed die and the cavity forming member of the movable dieforming a lens-molding cavity, and an advancement-retraction mechanismprovided between a second space-retaining plate of the space-retainingplates and the movable die plate for advancing and retracting themovable die plate relative to the first space-retaining plate, thelens-molding cavity being reduced when the movable die plate advancestoward the first space-retaining plate while the molding die is closed;closing the molding die and moving the cavity forming member of themovable die to a position where the thickness of the lens-molding cavityis greater than the thickness of an article to be molded; injecting athermoplastic molten resin into the lens-molding cavity established insetting the volume of the cavity and sealing in the molten resininjected into the lens-molding cavity; advancing the cavity formingmember of the movable die toward the cavity forming member of the fixeddie to pressurize the molten resin injected into the lens-moldingcavity; cooling the molten resin for a predetermined time aftercompletion of pressurizing the resin; and opening the movable die fromthe fixed die to eject a molding article, in which the thickness of themolding article is changed by adjusting at least one of the amount ofthe molten resin injected into the lens-molding cavity and thepressurizing force for pressurizing the molten resin inside thelens-molding cavity.

According to the above injection compression molding method of a lens,the molding die is closed during the cavity volume setting step and thecavity forming member of the movable die is moved to a position wherethe thickness of the lens forming cavity becomes a predeterminedthickness greater than the thickness of the article to be molded whilethe molding die is closed.

Subsequently, during resin injection seal-in step, the molten resin isinjected into the lens-molding cavity. Since the thickness of thelens-molding cavity is enlarged greater than the thickness of thearticle to be molded, inappropriate resin resistance against the moldingdie is not generated and injection and filling of the resin can besmoothly conducted during injection of the molten resin. The moltenresin injected into the cavity is sealed in the molding die. In otherwords, the molten resin is prevented from returning (backflowing) to theoutside of the molding die.

Next, during resin pressurizing step, the cavity forming member of themovable die is advanced toward the cavity forming member of the fixeddie. At this time, since the molten resin is filled and sealed in themolding die including the lens-molding cavity, the molten resin insidethe lens-molding cavity is pressurized. Incidentally, the resinpressurizing step may be started after, simultaneously with, orimmediately before completion of injecting and filling the resin.

Then, during the cooling step, the molten resin is cooled for apredetermined time after completion of resin pressurizing step. Themolten resin inside the lens-molding cavity is cooled while beingcompressed, and is gradually solidified and shrunk in accordance withthe progress in the cooling.

Lastly, in the die-releasing step, the movable die is opened relative tothe fixed die after completion of the cooling step, thereby ejecting themolding die.

In the above series of processes of the present invention, a moldingarticle of different thickness can be obtained by adjusting at least oneof the amount of the molten resin injected into the lens-molding cavityand the pressurizing force for pressurizing the molten resin inside thelens-molding cavity, so that it is not necessary to provide a pluralityof molding dies having cavity thickness corresponding to the thicknessof the final product as in a conventional arrangement, thereby easilychanging the lens thickness with low cost and no special additionalwork.

Specifically, in the above arrangement, since the molten resin injectedinto the lens-molding cavity is sealed in the molding die and isprevented from returning (backflowing) to the outside of the molding dieduring the resin injection seal-in step, molding article of differentthickness can be obtained by adjusting the amount of the molten resininjected into the lens-forming cavity during the resin injection seal-instep and/or the pressurizing force for pressuring the molten resininside the lens-molding cavity during the resin pressurizing step.

Incidentally, the above arrangement of the third aspect of the presentinvention may be combined with the arrangement of the first and/or thesecond aspect of the present invention.

An apparatus for performing the above-described invention may preferablybe arranged as follows.

An injection compression molding machine of a lens according to fourthaspect of the present invention includes: a tie bar for mutuallyconnecting a pair of space-retaining plates; a movable die plate movablealong the tie bar; a molding die provided between a firstspace-retaining plate of the pair of space-retaining plates and themovable die plate, the molding die accommodating a lens-molding cavityincluding a pair of cavity forming members for shaping concave andconvex surfaces of a lens; a toggle link mechanism provided between asecond space-retaining plate of the pair of space-retaining plates andthe movable die plate for advancing and retracting the movable die platerelative to the first space-retaining plate, the lens-molding cavitybeing reduced when the toggle link mechanism is actuated to advance themovable die plate toward the first space-retaining plate while themolding die is closed; a tie bar extension sensor for detecting theextension of the tie bar; and a space adjuster for adjusting thedistance between the pair of the space-retaining plates so that theextension of the tie bar detected by the tie bar extension sensorbecomes a desired value when the toggle link mechanism is moved to anextension limit thereof.

According to the above injection compression molding machine, since thetie bar extension sensor for detecting the extension of the tie bar, anda space adjuster for adjusting the distance between a pair ofspace-retaining plates so that the extension of the tie bar detected bythe tie bar extension sensor becomes a predetermined value when thetoggle link mechanism is moved to the extension limit are provided tothe injection compression molding machine for clamping the die by thetoggle link mechanism, the above-described injection compression moldingmethod of the present invention can be performed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration showing an injection compression moldingmachine applied with a method according to an embodiment of the presentinvention;

FIG. 2 is a cross section showing a molding die according to theaforesaid embodiment;

FIG. 3 is a cross section taken along III-III line in FIG. 2;

FIG. 4 is a cross section taken along IV-IV line in FIG. 2;

FIG. 5 is a flow chart showing a molding process of a lens in theaforesaid embodiment;

FIG. 6 is an illustration showing a relationship between lens power andclamping force in setting resin pressurizing condition in the aforesaidembodiment;

FIG. 7 is an illustration showing setting process of the resinpressurizing condition in the aforesaid embodiment;

FIG. 8 is an illustration showing a cavity in mechanical-end clampingduring resin pressurizing condition setting process in the aforesaidembodiment;

FIG. 9 is an illustration of an entire machine when the dies are closedat a parting line in the aforesaid embodiment;

FIG. 10 is an illustration of a cavity when the dies are closed at theparting line in the aforesaid embodiment;

FIG. 11 is an illustration showing the entire machine while settingcavity volume in the aforesaid embodiment;

FIG. 12 is an illustration showing the cavity while setting the cavityvolume in the aforesaid embodiment;

FIG. 13 is an illustration showing the entire machine while pressurizingthe resin in the aforesaid embodiment;

FIG. 14 is an illustration showing the cavity while pressurizing theresin in the aforesaid embodiment;

FIG. 15 is an illustration showing a relationship between clamping forceand extension of tie bar during cooling step in the aforesaidembodiment;

FIG. 16 is an illustration showing a relationship between clamping forceand extension of tie bar during the cooling step and a primarydie-releasing step in the aforesaid embodiment; and

FIG. 17 is a perspective view showing an obtained molding article in theaforesaid embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S)

An embodiment of the present invention will be described in detail belowwith reference to attached drawings.

[Machine Arrangement]

In the present embodiment, the injection compression molding method ofthe present invention is applied to an injection compression moldingmachine of a spectacles lens (meniscus-shaped spectacles lens:single-vision, multifocal, or progressive-power) of which entirearrangement is shown in FIG. 1. The material of the spectacles lensmolded therein is thermoplastic resin such as PMMA (polymethylmethacrylate) and, PC (polycarbonate).

The injection compression molding machine of the present embodiment hasa clamping device 60 having a molding die 50, an injection device 80 asan injection means for injecting plasticized and measured material resinto fill the molding die 50 and a die temperature adjuster 51 forcontrolling the temperature of the molding die at a predeterminedtemperature.

The clamping device 60 has a fixed die plate 61 and a rear plate 62 as apair of space-retaining plates, a plurality of tie bars 63 for mutuallyconnecting the fixed die plate 61 and the rear plate 62, a movable dieplate 64 movable along the tie bar 63, and a toggle link mechanism 65 asan advancement-retraction mechanism provided between the rear plate 62and the movable die plate 64 for advancing and retracting the movabledie plate 64 relative to the fixed die plate 61. Incidentally, themolding die 50 is provided between the fixed die plate 61 and themovable die plate 64.

The fixed die plate 61 is fixed on a frame 66.

The rear plate 62 is fixed to the frame 66 through a die thicknessadjuster 67. The die thickness adjuster 67 is a known device, which iscapable of moving the rear plate 62 toward and away from the fixed dieplate 61. The die thickness adjuster 67 constitutes a space adjuster foradjusting the distance between the fixed die plate 61 and the rear plate62 as the pair of space-retaining plates.

Further, since the rear plate 62 connected with the tie bar 63 isslidable through the die thickness adjuster 67, the rear plate 62 can beadjusted with the extension of the tie bar 63.

Further, an extension detecting sensor 68 as a tie bar extension sensorfor detecting the extension of the tie bar 63 is provided on the tie bar63.

The value detected by the extension detecting sensor 68 is displayed ona display (not shown) within a range of 0 to 1000με in the machine ofthe present embodiment. The extension of the tie bar 63 can be obtainedby the detected value (strain) of the extension detecting sensor 68shown on the display. Specifically, since 1με stands for 1 micro-strain(0.001 mm extension per 1 m), the extension of the tie bar 63 can becalculated based on the strain shown on the display.

The clamping force [ton] can be calculated based on the followingformula.Clamping force [ton]=με(displayed strain)*simplified coefficient

Incidentally, the simplified coefficient can be obtained by the elasticcoefficient, the diameter and the number of the tie bar.

The toggle link mechanism 65 has a plurality of toggle links 71A, 71Bstretched between the movable die plate 64 and the rear plate 62 andhaving an intermediate portion capable of being bent inwardly, a ballscrew 72 supported at the center of the rear plate 62 in parallel withthe tie bar 63 and rotated by a clamping motor (not shown), a crosshead73 screwed to the ball screw 72, and connection links 74A, 74Bconnecting the crosshead 73 with the bent portion of the respectivetoggle links 71A and 71B. Accordingly, when the crosshead 73 is advanced(rightward movement in FIG. 1), the toggle links 71A and 71B areextended through the connection links 74A and 74B. On the contrary, whenthe crosshead 73 is retracted (leftward movement in FIG. 1), the togglelinks 71A and 71B are bent inward through the connection links 74A and74B (i.e. the movable die plate 64 is retracted). Incidentally, thoughthe toggle-link mechanism has wide variety of mechanism such asdouble-link mechanism with complicated structure, basic arrangement isused as an example in the description of the present embodiment.

The injection device 80 has an injection cylinder unit 82 forplasticizing, kneading and measuring the material resin loaded from ahopper 81. The injection cylinder unit 82 accommodates a screw (notshown) thereinside and is attached with a long injection nozzle 85capable of going through into the molding die 50 at a distal endthereof. Incidentally, a band heater (not shown) is wound around theoutside of the injection cylinder unit 82.

The die temperature adjuster 51 is connected to respective components ofthe molding die 50 (insert, insert guide etc.) so thattemperature-controlling fluid can be supplied thereto. Specifically, thetemperature of the supplied temperature-controlling fluid is controlledso that the respective components of the molding die 50 has thetemperature set in accordance with the types of the lens to be molded.

FIG. 2 is a cross section of the molding die 50, FIG. 3 is a crosssection taken along III-III line in FIG. 2, and FIG. 4 is a crosssection taken along IV-IV line in FIG. 2.

As shown in FIG. 2, the molding die 50 has a movable die 1 and a fixeddie 2 separated in right and left direction at a parting line PL. Twospectacles-lens molding cavities 3, and a runner 49 connected to therespective spectacles-lens molding cavities 3 through a gate G toconnect the cavities 3 are formed between the movable die 1 and thefixed die 2. A sprue 48 formed by a sprue bush 47 is arrangedorthogonally with the runner 49. The two spectacles-lens moldingcavities 3, the runner 49 and the sprue 48 are included in a mold set45. Incidentally, the runner 49 and the sprue 48 form a resin channelinside the molding die 50.

Die body 4 of the movable die 1 is composed of an insert guide 5 and dieplates 6 and 7. Die body 8 of the fixed die 2 is composed of an insertguide 9 and a die plate 10. A spectacles-lens molding optical inserts 11and 12 as a cavity-forming member forming the cavity 3 is accommodatedinside the respective insert guides 5 and 9 in a manner slidable in adirection perpendicular to the parting line PL. Though not shown, atemperature-fluid circulation groove for circulating thetemperature-controlling fluid supplied by the die temperature adjuster51 is formed on the insert guides 5 and 9 and the inserts 11 and 12.

The die body 8 of the fixed die 2 is fixed to a die attachment member 15fixed on the fixed die plate 61. The die body 4 of the movable die 1 isconnected to a die attachment member 16 composed of a first member 16Aand a second member 16B with a bolt 17 shown in FIG. 3. A coned discspring 17A is interposed between the die body 4 and the die attachmentmember 16 as a resilient member inserted to the outer circumference ofthe bolt 17. The die attachment member 16 is fixed to the movable dieplate 64.

A gap S is provided between the die body 4 and the die attachment member16, and the die body 4 and the die attachment member 16 are capable ofbeing opened and closed in right and left directions by the gap S beingguided by a guide pin 18. In other words, by advancing the movable dieplate 64 after closing the die, the die attachment member 16 is pressedthrough the movable die plate 64 to close the gap S.

A hydraulic cylinder 19 is provided to the die attachment member 16. Apiston rod 21 connected to a piston 20 of the hydraulic cylinder 19penetrates an inside of a back insert 22 fixed to an end of thehydraulic cylinder 19 and is provided with a T-shaped clamp member 23 ata tip end thereof. The T-shaped clamp member 23 is engaged with aT-shaped groove 24 formed on the other end of the insert 11 in anengageable and detachable manner. Accordingly, the insert 11 can beexchanged.

A hydraulic cylinder 26 is provided on the die attachment member 15. Apiston rod 28 connected to a piston 27 of the hydraulic cylinder 26penetrates the inside of the die attachment member 15 and is providedwith a T-shaped clamp member 29 at a tip end thereof. The T-shaped clampmember 29 is engaged with a T-shaped groove 30 formed on the other endof the insert 12 in an engageable and detachable manner. Accordingly,the insert 12 can be exchanged.

A pressure-receiver 32 is fixed on the other end of the hydrauliccylinder 19. When the pressure-receiver 32 is pressed by an eject rod 34inserted from a hole 33 formed on the die attachment member 16, thehydraulic cylinder 19, the back insert 22 and the insert 11 are alsopressed, so that the lens formed in the cavity 3 is ejected when themovable die 1 and the fixed die 2 are parted.

An eject pin 35 is located at the center of the movable die 1 and thedie attachment member 16 in a manner capable of advancement andretraction in right and left directions. A pressure-receiver 36 beingdisplaceable in right and left directions at a predetermined stroke isfixed on an end of the eject pin 35. When the pressure-receiver 36 ispressed by an eject rod 38 inserted from a hole 37 formed on the dieattachment member 16, the eject pin 35 is pushed out.

Incidentally, spring force of a spring 40 wound around the outercircumference of an eject return pin 39 works leftward (in the figure)on the pressure-receiver 32. Similarly, as shown in FIG. 4, spring forceof a spring 42 wound around the outer circumference of an eject returnpin 41 works leftward (in the figure) on the pressure-receiver 36 andthe pressure-receiver 36 is positioned at a predetermined location.Accordingly, when the eject rods 34 and 38 are retracted, thepressure-receivers 32 and 36 are also retracted to restore the originalposition thereof.

A nozzle shut mechanism 90 has a nozzle shut pin 91 as a shutting memberas shown in FIG. 4. The nozzle shut pin 91 is inserted to the sidewallof the sprue bush 47 in an advanceable and retractable manner in adirection approximately perpendicular to the central axis of the spruebush 47 and the rear end thereof is fixed to a piston rod 94 of ahydraulic cylinder 93 through a connection piece 92. The hydrauliccylinder 93 is fixed to the die attachment member 15 through a cylinderattachment plate 95. The backflow of the resin can be prevented bysliding the nozzle shut pin 91 to shut the distal opening of theinjection nozzle 85 while the injection nozzle 85 is pressed to thesprue bush 47 (see Japanese Utility-Model Laid-Open Publication Hei6-9826).

[Lens Molding Process]

Initially, the inserts 11 and 12 are exchanged in accordance with thetype of the lens to be molded. When the inserts 11 and 12 are exchanged,the movable die 1 including the die attachment member 16 is retracted tobe parted from the fixed die 2. Further, the piston rod 21 of thehydraulic cylinder 19 is advanced and the piston rod 28 of the hydrauliccylinder 26 is advanced, so that the T-shaped clamp members 23 and 29attached to the tip end of the piston rods 21 and 28 are projected fromthe insert guides 5 and 9.

Another inserts 11 and 12 to be newly attached to the die bodies 4 and 8of the movable die 1 and the fixed die 2 are horizontally transferredwhile holding with a robot arm (not shown) and the T-shaped grooves 24and 30 of the inserts 11 and 12 are engaged with the T-shaped clampmembers 23 and 29. Subsequently, the piston rod 21 of the hydrauliccylinder 19 is retracted to draw in the insert 11 and the piston rod 28of the hydraulic cylinder 26 is retracted to draw in the insert 12, sothat the inserts 11 and 12 are fitted to the insert guides 5 and 9.

Accordingly, the insert is exchanged with an insert capable of forming acavity 3 having thicker central portion than the peripheral portionthereof for molding a plus lens and to an insert capable of forming acavity 3 for forming a cavity 3 having thinner central portion than theperipheral portion thereof.

The molding process of a spectacles lens having meniscus-shape is shownin the flowchart of FIG. 5.

In ST (step) 1, resin pressurizing condition is set for adjustingclamping force in advance in accordance with the characteristics of thelens to be molded (such as lens shape and lens power) to applyappropriate pressure to the resin inside the cavity 3. For instance, asshown in FIG. 6, appropriate clamping force corresponding to lens poweris set, based on which the resin pressurizing condition is set. Ofcourse, it is obvious that the resin pressurizing condition is changedin accordance with the characteristics of the lens resin, which has tobe considered in all the molding condition.

Initially, as shown in FIG. 7(A), the molding die 50 is closed until theparting line PL is brought into close contact. Specifically, when thecrosshead 73 of the toggle link mechanism 65 is advanced toward themovable die plate 64, the toggle links 71A and 71B are stretched to movethe movable die plate 64 toward the fixed die plate 61, so that the diesare closed retaining a gap S (S1) while the coned disc spring 17Ainterposed between the die body 4 of the movable die 1 and the dieattachment member 16 is not compressed.

Subsequently, as shown in FIG. 7(B), when the crosshead 73 is furtheradvanced to an origin (zero position), the toggle links 71A and 71B arestretched to the maximum thereof. Then, the tie bar 63 is extended togenerate a clamping force. At this time (mechanical-end clamping), thecavity 3 is set smaller than the volume (thickness) of the lens to bemolded as shown in FIG. 8. In other words, the cavity 3 is set smallerthan the ejected molding product (thickness) during mechanical-endclamping process.

The nearer the crosshead position is located when the dies are closedwhile the coned disc spring 17A is not compressed relative to themovable die plate 64, the more the toggle links 71A and 71B arestretched, where the tie bar 63 is less stretched and the clamping forceis weaker. In other words, since the crosshead position when the diesare closed is determined by the space between the fixed die plate 61 andthe rear plate 62, the clamping force is set by adjusting the positionof the rear plate 62 by the die thickness adjuster 67.

Here, as shown in FIGS. 7(A) and 7(B), the clamping force is set bycontrolling the extension ΔL during mechanical-end clamping (conditionshown in FIG. 7(B)) relative to the space between the fixed die plate 61and the rear plate 62 when the molding die 50 is closed at the partingline PL (condition shown in FIG. 7(A)).

In ST2, the resin is measured. In the injection device 80, the materialresin loaded into the hopper 81 is plasticized and the plasticizedmolten resin is introduced into the injection cylinder unit 82 formeasurement. Here, the molten resin necessary for the mold set 45 havingthe two lens-forming cavities 3, the runner 49 and the sprue 48 ismeasured.

In ST3, the dies are closed at the parting line PL. As shown in FIG. 9,when the crosshead 73 of the toggle link mechanism 65 is advanced, thetoggle links 71A and 71B are stretched to move the movable die plate 64toward the fixed die plate 61, and the molding die 50 is closed at theparting line PL retaining the gap S while the coned disc spring 17Ainterposed between the die body 4 of the movable die 1 and the dieattachment member 16 is not compressed. In this condition, the gap S isset at the maximum (S1). Further, the cavity 3 is set wider than the sumof the thickness of the molding article and enlargement as shown in FIG.10.

In ST4, the cavity volume is set. As shown in FIG. 11, when thecrosshead 73 is advanced to a predetermined position (cavity volumesetting position), the toggle links 71A and 71B are stretched toward thefixed die plate 61 to move the movable die plate 64 to a cavityenlargement position. The cavity enlargement amount is determinedaccording to the setting of the crosshead position. Accordingly, the gapS of the molding die 50 is reduced retaining the cavity enlargementamount (gap S3). At this time, the volume (thickness) of the cavity 3 isgreater than the volume (thickness) of the lens to be molded, i.e. thethickness of the molding article to be ejected. Further, since the coneddisc spring 17A is compressed, a little clamping force is generated as areaction force thereof (accordingly, the space between the fixed dieplate 61 and the rear plate 62 is L′).

In ST5, the resin is injected. The molten resin measured during themeasurement process is injected to the mold set 45 through the channelof the injection nozzle 85. Specifically, the molten resin introducedinto and measured in the injection cylinder unit 82 of the injectiondevice 80 is injected by rotating a screw. Then, the molten resin isgradually filled into the cavity 3 through the injection nozzle 85, thesprue 48 of the sprue bush 47, the runner 49 and the gate G. When theresin is filled in the cavity 3, the injection speed is controlled to beconstant. Since the cavity 3 is greatly enlarged, injection and fillingprocesses are conducted without generating inappropriate resinresistance against the molding die 50.

In ST6, the resin is sealed in the die. The advancement of the crosshead73 is immediately started just before completing to inject and fill themolten resin, and the injection nozzle 85 is closed by the nozzle shutmechanism 90 immediately after completion of injecting and filling.Specifically, the nozzle shut pin 91 is projected into the sprue 48 toclose the distal end of the channel of the injection nozzle 85.Accordingly, the molten resin is sealed in the molding die 50.

In ST7, the resin is pressurized. After the crosshead 73 startsadvancement in ST6, when the crosshead 73 advances to the origin(zero-position) to be stopped, the toggle links 71A and 71B arestretched to the maximum thereof, so that the molten resin sealed in themolding die 50 is compressed and pressurized.

At this time, as shown in FIG. 13, the cavity volume (thickness) of themolding die 50 when the molding die 50 is closed to mechanical endthereof is set smaller than the volume (thickness) of the lens to bemolded, so that the molten resin necessary for the lens to be molded canbe filled and the gap S of the molding die 50 is not completely closedto the mechanical end even after compression. Specifically, on accountof the gap S4, the insert 11 in contact with the die attachment member16 directly pushes the resin inside the cavity 3. Accordingly, the resininside the cavity 3 is compressed to a thickness between the thicknessof the ejected molding article and the thickness during mechanical-endclamping as shown in FIG. 14.

Here, the extension of the tie bar 63 indicating the pressure forcompressing the resin inside the cavity 3 is a sum (ΔL+α) of theextension (ΔL) of the tie bar 63 on account of clamping force and theextension (α) of the tie bar 63 due to incomplete closing of the moldingdie 50 on account of the resin volume inside the cavity 3. Accordingly,in order to apply appropriate pressure to the resin inside the cavity 3,the mutually relating resin volume and the clamping force may beappropriately determined in accordance with the characteristics of thelens.

Incidentally, since the molten resin injected into the cavity 3 duringthe process for injecting and sealing the resin is sealed in the moldingdie 50 and is prevented from returning (backflowing) to the outside ofthe molding die 50, molding articles of different thickness can beobtained by adjusting the amount of the molten resin injected into thecavity 3 during the resin injection seal-in step and/or the pressurizingforce for pressurizing the molten resin inside the cavity 3 during theresin pressurizing step.

In the present embodiment, the amount of the molten resin injected intothe cavity 3 (the amount necessary for the mold set 45 having twolens-molding cavities 3, the runner 49 and the sprue 48) during theresin injection-seal-in process, i.e. the amount of the molten resin forobtaining desired thickness of the molding article, is measured duringmeasurement step in ST2.

In ST8, the cooling step is conducted. Specifically, the temperature ofthe temperature-controlling fluid is controlled by the die-temperatureadjuster 51 so that the respective components (such as insert and insertguide) of the molding die 50 becomes a predetermined temperature belowTg in accordance with the characteristics of the lens to be molded.

The resin filled in the cavity 3 is gradually solidified and shrunk inaccordance with the progress in cooling while being compressed. Themovable die plate 64 gradually advances in accordance with the shrinkageof the resin. When the movable die plate 64 advances, the space betweenthe fixed die plate 61 and the rear plate 62 are reduced, i.e. theelasticity of the tie bar 63 is recovered, so that the pressure appliedto the resin inside the cavity 3 is lowered.

FIG. 15 shows a relationship between the clamping force during thecooling step and the extension of the tie bar 63. In the figure, pointP1 indicates a preset condition of the clamping force which is set inaccordance with lens characteristics (aspherical single-vision lens,lens power −4.00 D, diameter 76 mm, central thickness of molded lens 1.1mm) in advance to injection. The dies are once opened (point P2), andthe filling process of the molten resin is started and the dies areclamped, where the clamping force becomes the maximum at a point P3indicating a point where the toggle links 71A and 71B are fullystretched.

As shown in the figure, the clamping force (pressure applied to theresin inside the cavity 3) and the extension of the tie bar 63 exhibitsthe same tendency, which are slightly lowered at the initial stage ofthe cooling step (P3 to P5 section in FIG. 15) but are extremely gentlylowered (hardly changed) thereafter (P5 to P6 section).

The section from P6 to P10 is initial stage of die-releasing step(primary step), during which the extension of the tie bar is controlled.In the present invention, it is found that the control during thesection greatly influences on the separation failure and lens quality.

In the section from P6 to P8, the clamping force is gradually lowered tocontrol the restoration of the elasticity of the tie bar by graduallylowering the clamping force, and the change in the clamping force isincreased at the section from P8 to P10 to recover the elasticity.

Thereafter, the subsequent sections continues to an ordinary actualdie-releasing step where the movable plate is further retracted toreduce the clamping force and the dies are opened at a stretch (see U.S.Pat. No. 5,855,824 by the Applicant of the present application).

Incidentally, in order to clarify the border between the cooling stepand the die-releasing step, the points P are specified instead ofindicating a region.

The relationship between restoration of the elasticity of the tie bar 63caused by decrease in the clamping force and the cooling time can beobtained by experimentation. Accordingly, the clamping force iscontrolled by determining an appropriate cooling time considering thematerial used and the lens characteristics.

Specifically, the cooling time may preferably lengthened in accordancewith increase in the lens power, the outer diameter of the lens and thelens thickness.

However, the cooling time may of course be greatly changed consideringthe lens material.

In ST9, die-release operation is conducted. During the die-releaseoperation, the crosshead 73 of the toggle link mechanism 65 is retractedtoward the rear plate 62. When the crosshead 73 is refracted, the togglelinks 71A and 71B stretched to the maximum thereof is contracted, sothat the elasticity of the tie bar 63 is recovered.

The movable die plate 64 connected with the toggle link 71A and 71B ismoved in a direction for the molding die 50 to be opened, i.e. towardthe rear plate 62 in accordance with the retraction of the crosshead 73.However, since the rear plate 62 moves toward the fixed die plate 61 dueto the restoration of the elasticity of the tie bar 63, the movement ofthe movable die plate 64 and the contraction of the tie bar 63 due tothe recovery of elasticity cancel each other, so that the crosshead 73continues to be retracted without significant change in the spacebetween the fixed die plate 61 and the movable die plate 64 (i.e. therelative position between the insert 12 of the fixed die 2 and theinsert 11 of the movable die 1).

FIG. 16 shows a relationship between the clamping force and the dieplate space (the space between the fixed die plate 61 and the movabledie plate 64) during the cooling step (the section between S3 and S6,P3′ and P6′) and the primary die-releasing step (the section between S6and S10, P6′ and P10′). As can be seen from the graph, though theclamping force (the pressure applied to the resin inside the cavity 3)rapidly decreases when entering the primary die-releasing step (thesection between the P6′ and P10′) after completion of the cooling step,the space between the fixed die plate 61 and the movable die plate 64does not substantially change during the primary die-release operation(the section between S6 and S10). Here, the change in the space betweenthe fixed die plate 61 and the movable die plate 64 (i.e. the spacebetween the insert 12 of the fixed die 2 and the insert 11 of themovable die 1) is maintained within the difference between the thicknessof the lens-molding cavity during compression and the thickness of theejected molding article (e.g. not more than 0.3 mm in the presentembodiment).

Accordingly, though the pressure applied to the resin is reduced at theprimary die-releasing step, the relative position between the insert 11of the movable die 1 and the insert 12 of the fixed die 2 is keptsubstantially constant, so that the pressure applied to the resin insidethe die is gently and smoothly reduced, thereby eliminating thedeformation of the molding article during die-releasing step to obtain ahighly accurate lens molding even when the cooling time is shortened.

Since the pressure applied to the resin inside the cavity 3 can becontrolled by adjusting the retraction speed of the crosshead 73, thedecrease in the pressure applied to the resin inside the cavity 3 can becontrolled by adjusting the retraction speed of the crosshead 73. Theappropriate relationship between the retraction of the crosshead 73 andthe elasticity recovery of the tie bar 63 is obtained in advance byexperimentation, so that the control condition of the crosshead 73 canbe determined in accordance with the material to be used and the lenscharacteristics.

In ST10, molding article is ejected. When the crosshead 73 is retractedto the maximum, the space between the movable die plate 64 and the fixeddie plate 61 becomes the maximum, and the molding die is split andopened at the parting line PL, where the molding article is ejected andis separated from the molding die 50.

A molding article 101 shown in FIG. 17, for instance, can be obtainedaccording to the above series of processes. The molding article 101 isformed of spectacles lenses 102 molded by the two lens-molding cavity 3,a connecting portion molded by the runner 49 for connecting the twospectacles lenses 102, and a rod-shaped portion 104 molded by the sprue48 extending perpendicular to the central portion of the connectingportion 103 in a thickness direction of the lens 102. The lens 102 isimmersed in abrasion resistant hard coating liquid and, thereafter, thelens 102 and the connecting portion 103 are cut by a cutter.Accordingly, two spectacles lenses 102 coated by hard-coat film of thehard-coat fluid can be simultaneously obtained from a single moldingarticle 101.

Effects of the Embodiment

The cavity volume (thickness) is set thicker than the thickness of themolding article in the cavity-volume setting process and the moltenresin is injected into the cavity 3 and sealed therein in the resininjection seal-in step. The toggle link mechanism 65 is stretched toextend the tie bar 63 during the resin pressurizing step while the resinis sealed in the molding die 50 and the resin cooling step is conductedwhile applying the reaction force generated by the extension of the tiebar 63 to the resin inside the cavity 3, so that sink mark and strain ofthe molding article and molding cycle can be reduced.

Specifically, when the molten resin inside the cavity 3 is cooled for apredetermined time during the cooling step, the molten resin inside thecavity 3 is gradually solidified and shrunk in accordance with theprogress in the cooling while being compressed. The movable die plate 64gradually advances in accordance with the shrinkage of the resin. Whenthe movable die plate 64 advances, the space between the fixed die plate61 and the rear plate 62 is lessened, i.e. the elasticity of the tie bar63 is recovered, so that the pressure applied to the molten resin insidethe cavity 3 is gradually lowered. Accordingly, as the molten resininside the cavity 3 is gradually cooled and solidified to be shrunk, thepressure applied to the molten resin inside the cavity 3 can begradually and gently lowered through the advancement of the movable dieplate 64 and the elasticity recovery of the tie bar 63, so that theobtained entire molding article can be uniformly cooled. As a result,sink mark and strain can be reduced, and cooling time, the entiremolding cycle as well, can be shortened. For reference, according to themethod of the present embodiment, the die-releasing step can be startedafter eighty seconds of cooling time for molding a lens of 76 mmdiameter and −4.00 D (diopter) lens power.

In the die-releasing step, after completing the cooling step, thepressure applied to the molten resin inside the lens-molding cavity 3 islowered for a predetermined time while the relative position between theinsert 11 of the movable die 1 and the insert 12 of the fixed die 2 ismaintained substantially constant, and the movable die 1 is partedrelative to the fixed die 2. In other words, at the primary die-releaseoperation, since the relative position between the insert 11 of themovable die 1 and the insert 12 of the fixed die 2 is kept substantiallyconstant (where the change in the space between the insert 11 of themovable die 1 and the insert 12 of the fixed die 2 is within thedifference between the thickness of the lens-molding cavity beingcompressed and the thickness of the ejected molding article) even afterthe pressure applied to the resin inside the lens-molding cavity 3 islowered, the deformation of the molding article can be restrained.

Since the relative position of the insert 11 of the movable die 1 andthe insert 12 of the fixed die 2 is kept substantially constant in spiteof the fact that the pressure applied to the resin is lowered, highlyaccurate lens molding with less deformation can be obtained in thedie-releasing step even when the cooling step is shortened. Accordingly,the molding cycle can also be shortened while obtaining highly accuratelenses.

The toggle link mechanism 65 is used as a mechanism for advancing andretracting the movable die plate 64 relative to the fixed die plate 61,the toggle link mechanism 65 being stretched to close the molding die50. Further, the distance between the fixed die plate 61 and the rearplate 62 is adjusted so that a desired extension of the tie bar 63 canbe obtained while the toggle link mechanism 65 is actuated to the moststretched position to set the volume of the cavity 3 at the minimumvolume (minimum thickness), thus facilitating the operation during theresin pressurizing step.

Specifically, since the distance between the fixed die plate 61 and therear plate 62 is adjusted so that the extension of the tie bar 63becomes a predetermined desired value (ΔL, for instance) when the volumeof the cavity 3 is set minimum by driving the toggle link mechanism 65to the extension limit thereof, it is only necessary for the toggle linkmechanism 65 to be moved to the extension limit thereof during the resinpressurizing step. The extension of the tie bar 63 during the resinpressurizing step is equal to the sum of the extension (ΔL) during theclamping force adjusting step and the extension (α) of the tie bar 63caused on account of incomplete closing of the molding die 50 due to theresin volume inside the cavity 3. Accordingly, the setting the extension(ΔL) considering the extension (α) during the clamping force adjustingstep, it is only necessary for the toggle link mechanism 65 to be movedto the extension limit during the resin pressurizing step.

Since the rear plate 62 is moved toward and away from the fixed dieplate 61 during the clamping force adjusting step, conventional diethickness adjuster 67 can be used for moving the rear plate 62 towardand away from the fixed die plate 61.

Since the nozzle shut pin 91 is projected into the sprue 48 to shut thenozzle channel for injecting the thermoplastic resin in shutting theresin filled inside the cavity 3, it is only necessary to project thenozzle shut pin 91 to the nozzle channel and the thermoplastic resin canbe shut inside the cavity 3 immediately after completing injection ofthe molten resin inside the cavity 3. Accordingly, backflow of the resincan be prevented even when the resin pressurizing step is startedimmediately before completing injection.

According to the present embodiment, since a molding article ofdifferent thickness can be obtained by adjusting at least one of theamount of the resin injected into the inside of the cavity 3 during theresin injection seal-in step and pressurizing force for pressurizing themolten resin inside the cavity 3 during the resin pressurizing step, itis not necessary to prepare a plurality of molding dies having cavitythickness corresponding to the thickness of the final molding article inorder to obtain molding article of different thickness, so thatproduction cost can be reduced and the thickness of the lens can beeasily changed without requiring special additional work.

Specifically, since the molten resin injected into the inside of thecavity 3 is sealed in the molding die 50 during the resin injectionseal-in step and is prevented from returning (backflowing) to theoutside of the molding die 50, molding article of different thicknesscan be obtained by adjusting the amount of the resin injected into theinside of the cavity 3 during the resin injection seal-in step and/orthe pressurizing force for pressurizing the molten resin inside thecavity 3 during the resin pressurizing step.

MODIFICATIONS

The scope of the present invention is not restricted to the arrangementand method described in the above embodiment, but includes followingmodifications.

Though the toggle link mechanism 65 is used for advancing and retractingthe movable die plate 64 relative to the fixed die plate 61, sucharrangement is not limiting. Any mechanism provided between the rearplate 62 and the movable die plate 64 for moving the movable die plate64 toward and away from the fixed die plate 61 may be employed as longas the relative position between the rear plate 62 and the movable dieplate 64 can be held constant when the movable die plate 64 is movedtoward the fixed die plate 61 and the extension of the tie bar becomes apredetermined value.

Though the mold set 45 includes the two lens-forming cavities 3 in theabove-described embodiment, only one lens-molding cavity or more thantwo lens-forming cavities may be provided.

Though the compression is started immediately before completion of theinjection of the molten resin in the above-described embodiment, thecompression process may be started simultaneously with or aftercompletion of the molten resin according to the type of the lens to bemolded (minus lens and plus lens).

Though the rear plate 62 is moved toward and away from the fixed dieplate 61 for adjusting the space between the rear plate 62 and the fixeddie plate 61 in the above embodiment, reverse arrangement is possible.

Though the injection compression molding machine of a spectacles lens isused as an example in the above-described embodiment, the moldingarticle is not restricted to a spectacles lens but other general lenscan be molded.

1. An injection compression molding method of a lens for molding a lensof a thermoplastic resin, comprising the steps of: providing a moldingmachine having a tie bar for mutually connecting a pair ofspace-retaining plates, a movable die plate movable along the tie bar, amolding die provided between a first space-retaining plate of the pairof the space-retaining plates and the movable die plate and having afixed die and a movable die moving toward and away from the fixed die,the fixed die and the movable die respectively having a cavity formingmember for shaping concave and convex surface of the lens, the cavityforming member of the fixed die and the cavity forming member of themovable die forming a lens-molding cavity, and an advancement-retractionmechanism provided between a second space-retaining plate of thespace-retaining plates and the movable die plate for advancing andretracting the movable die plate relative to the first space-retainingplate, the lens-molding cavity being reduced when the movable die plateadvances toward the first space-retaining plate while the molding die isclosed; closing the molding die and moving the cavity forming member ofthe movable die to a position where the thickness of the lens-moldingcavity is greater than the thickness of an article to be molded;injecting a thermoplastic molten resin into the lens-molding cavityestablished in setting the volume of the cavity and sealing in themolten resin injected into the lens-molding cavity; advancing the cavityforming member of the movable die toward the cavity forming member ofthe fixed die to pressurize the molten resin injected into thelens-molding cavity; cooling the molten resin for a predetermined timeafter completion of pressurizing the resin; and opening the movable diefrom the fixed die to eject a molding article, wherein the thickness ofthe molding article is changed by adjusting at least one of the amountof the molten resin injected into the lens-molding cavity and thepressurizing force for pressurizing the molten resin inside thelens-molding cavity.
 2. The injection compression molding method of alens according to claim 1, wherein the thickness of the molding articleis changed by adjusting at least one of the amount of the molten resininjected into the lens-molding cavity and the pressurizing force forpressurizing the molten resin inside the lens-molding cavity.